Editor's note: This paper was originally presented at SIL's General
CARLA Conference, 14-15 November 1996, Waxhaw, NC. CARLA, for Computer-Assisted
Related Language Adaptation, is the application of machine translation techniques
between languages that are so closely related to each other that a literal
translation can produce a useful first draft.

This paper documents the current status of a computational implementation of
English syntax according to the Government and Binding Theory. The program used
is PC-PATR (see McConnel 1995), based on the PATR formalism.
Both the relevant GB theory and the details of the implementation are covered in the text.
The appendices include sample trees and features and a list of example sentences.

Some current syntactic theories, such as Lexical Functional Grammar (LFG)
(Bresnan 1982), Generalized Phrase Structure Grammar
(GPSG) (Gazdar, Klein, Pullum, and Sag 1985) and
Head-Driven Phrase Structure Grammar (HPSG) (Pollard
and Sag 1987, 1994), were developed
with computer implementation in mind. They all employ feature unification
as an essential ingredient in their theory. Government and Binding Theory
(GB), on the other hand, was developed totally independently of any computational
concerns. LFG, GPSG, and HPSG have all been modeled to at least some extent
via the PATR formalism (Shieber 1986) upon which
PC-PATR is founded. To my knowledge, no such implementation has ever been
done for GB. This paper documents the current status of an implementation
of most of GB syntax using the PC-PATR program (McConnel 1995).

The purpose of this experimental implementation is two-fold. Initially, it
can be used as a teaching tool for both GB theory and for PC-PATR. The goal
is ultimately to develop a full grammar for Zapotec (and later other languages)
to be used as a syntactic parser and disambiguator for adaptation.

The major challenge that presented itself was how to implement a derivational
or multistratal model within a single stratum system. Since the surface string
is what is available to parse, S-structure is modeled by the grammar. GB's
traces allow the underlying D-structure to be seen within the S-structure,
and constraints between the two levels are captured via feature passing.
In each sub-theory within GB, what part of the theory could be modeled
computationally and how to do so within the current capabilities of PC-PATR
had to be determined.

The paper is divided into five basic sections: Phrase Structure and
Subcategorization, Question Formation, Constructions Involving Movement to
Subject Position, Case and Agreement, and Binding of Reflexives and Equi
Constructions. Within each section, the relevant GB theory is discussed first,
then how it was implemented in PC-PATR is
covered.[1] Since the English grammar is the
most complete, it is the focus of the sections on the PC-PATR implementation,
except where the work on another language provides additional insights. Two
appendices show the trees and feature structures for two complex examples
and a list of the sentences that the grammar accounts for so
far.[2] These sentences include many ungrammatical
examples that the grammar must rule out to be parsing correctly.

X-Bar Theory assumes that all phrases (including sentences and clauses) have
the same structure, made up of heads and their projections. The general rules
for this phrase structure are covered in section
2.1.2, after the importance of subcategorization
is discussed.

A word, such as a noun, verb, adjective
or preposition is a lexical category. In structural terms, they are called
heads. A particular head is choosy about what can combine with it
to form a phrase.[3]

A complement is a phrase that a head takes or selects or
subcategorizes for. Which complements
a particular verb takes is an arbitrary property of that verb. Adjectives,
nouns, and prepositions also subcategorize for their complements.

Heads and complements are not the only parts of phrases. For example, NPs
can be preceded by words (or sometimes phrases) like: the, no, some, every,
John's, my mother's. APs can be preceded by degree words such as: very,
extremely, rather, quite. PPs can be preceded by just, right.
These items, called specifiers, differ from complements in their position
in the phrase and in the fact that they are not subcategorized for by a
particular head.

Subcategorization is built into X-Bar Theory by the claim that a head
(=X0) subcategorizes for all and only its sisters. The complements
are always phrases. In addition, heads and their maximal projections share
some features, allowing heads to subcategorize for the heads of their sisters
(e.g. rely). Heads and their specifiers may also share some features.

The only other phrase structure rules allowed are an unordered, recursive
adjunction rule (20) (used for adjective phrases and
relative clauses modifying nouns, and adverbial phrases) and a general
conjunction rule (21):

A sample X-Bar tree is given in (22), where afraid
is the head, extremely is the specifier, and of very big mean dogs
is the complement in the AP. The APs very big and mean modifying
the noun dogs illustrate the recursive adjunction rule.

Each of the main verbs not only takes a clausal complement, but chooses which
complementizer the clause must have. This is reminiscent of a verb like
rely which subcategorizes for a PP complement which must have
on as the preposition. Only heads can subcategorize, and only complements
or the head of a complement may be subcategorized for. Therefore, the
complementizer must be the head of the complement clause, which is a
complementizer phrase or CP in X-Bar terms.

To reanalyze the structure of the sentence in X-Bar terms, its head, complement,
and specifier must be determined. Neither of the constituents on the right
side of the rule (23ii) can be the head of a phrase because
they are phrases themselves, not lexical items or words. Evidence of
subcategorization is again present in data like
(27)-(29).

When the complementizer is either that or whether, the sentence
that follows is a regular finite sentence. In contrast, when the complementizer
is for(28), to must be present followed
by the bare form of the verb. The complementizers that and
whether subcategorize for a finite complement, whereas for
requires a nonfinite complement.

The marker for nonfinite clauses in English is to. Since the
complementizer for subcategorizes for a nonfinite complement,
to must be the head. Further evidence that to is a head comes
from the fact that to subcategorizes for the bare form of the verb
following it.

Chomsky (1986) posits that the tense and agreement
features fill the same head position in finite sentences that to fills
in nonfinite sentences. The category is therefore called Inflection, or Infl
or I for short. This means a sentence is an IP.

I[+fin] is never filled by a lexical word at
D-structure in English.[6] It always takes
a VP as its complement, just as nonfinite to does. The subject NP
is assumed to fill the specifier position in the IP.

The PC-PATR implementation models the general
rules of X-Bar Theory with constrained phrase structure rules. Verbal
subcategorization requirements are implemented using a linked list within
the subcat feature, as shown in section
2.2.2.

PC-PATR provides the capability to write general phrase structure rules which
can be constrained via unification constraints. Nothing within PC-PATR limits
the rules to X-Bar Theory; the person writing the rules must discipline himself
to stay within the theory they are modeling.

The phrase structure rules in the English model of GB in PC-PATR are all
specific instances of the X-Bar rules for a particular category. Each rule
is constrained to assure that the rule is only used for the correct lexical
items and to pass certain features between mother and daughter nodes by requiring
that they unify. For example, the rule for the subtree including the nonfinite
marker to is given in (31).

The rule says that the intermediate projection I' splits into the head I
and its complement VP. The first four unification constraints require feature
sharing between the mother I' and a particular one of the daughters. The
[subcat], [moved], and [bind] features
(to be explained later) on the VP are passed up to the I' and must unify
with any value for those features present on I' due to another rule. The
[type] features on I' must unify with those on I. From the next
two constraints, we see that [type] must include both
[fin:-] and [q:-]. The last two constraints restrict
the [vform] and [person] features on the VP
appropriately for a nonfinite clause.

The only PC-PATR phrase structure rules that do not directly fit the X-Bar
rules are the rules which introduce subcategorized complements for verbs.
Rather than having all the complements be sisters of V0 under
V', the complements are each sisters of separate instances of V', in a stacked
structure generated by rules such as:[7]

(32)

a. V' = V'_2 NP
b. V' = V'_2 PP
c. V' = V

This structure allows the subcategorization requirements to be represented
in the form of a linked list (introduced by Shieber
1986:29), as discussed in the next section. It also has the benefit that
fewer phrase structure rules are needed than if each different ordering of
complements required a separate rule. For example, (32a)
is called twice for a verb taking two NPs; (32a) then
(32b) is used for a verb taking an NP and a PP; and two
instances of (32b) are needed for a verb subcategorizing
for two PPs. Subcategorized complements are still distinguished from adjuncts
in the PC-PATR implementation, because the adjuncts are adjoined to VP and
must not be subcategorized for.

To illustrate the difference, the GB tree for the VP throw the ball to
Bill is given in (33a) and the PC-PATR stacked structure
is shown in (33b).

The subcategorization information for a particular verb is encoded in the
\f line in the lexicon with an abbreviation for a template.
This template is then expanded in the grammar file into a feature structure.
The part of the feature structure relating to subcategorization is under
the feature [subcat], which is broken down into the subfeatures
[subj] and [comp] for 'subject' and 'complements',
respectively.[8] The information within
[subj] and [comp] has the form of a linked list.
This allows the complements to be matched one at a time with the proper category
in the phrase structure rule as the list is stepped through.

For example, the subcategorization information for a ditransitive verb like
throw would be encoded as shown in (34).

Given the subcat feature structure in (34) and the
constrained rules in (35), the PC-PATR program generates
the tree (33b) from the bottom up. The verb throw
is marked in the lexicon as ditransitive, so the feature structure in
(34) is assigned to it. The verb is placed into the tree
by rule (35c). The unification constraint on this rule
passes the subcat information to the mother node V'. The next two words the
ball would be separately parsed into an NP. The sequence V' NP matches the right side of rule (35a). The
first unification constraint requires that the features in <V'_2
subcat comp first>, which are [cat:NP], unify with
the features on the NP, as they do. The second unification constraint then
serves to step the list down one, passing only the remaining part of the
complement subcat information up to the mother V'. The third unification
constraint percolates the subject subcat information. Thus, the subcat feature
structure for the new V' is:

The process just discussed for the NP complement is repeated for the PP
complement with rule (35b). After stepping down again
through the complement subcat information, the feature structure passed to
the new mother V' allows no more complements, as shown in
(37).

Not all surface sentences are the same as their
underlying form: Yes/No questions and content questions are good examples.
GB proposes a movement account which is restricted by certain principles
and leaves behind a coindexed trace. The S-structure after movement is modeled
in PC-PATR using null elements for traces and feature passing to match the
subcategorization requirements appropriately.

The relationship between a declarative sentence, such as
(38a) and a Yes/No question, such as
(38b), is taken to be a clear case of head movement
in GB. (38c-f) show that more is going on in Yes/No questions
than simply putting a verb in front of the subject.

In Yes/No questions, this order and form restriction still holds. Then, the
highest auxiliary moves in front of the subject. Two other phenomena show
that the highest auxiliary is important also: VP deletion and the placement
of negation. In (40) negation follows the first auxiliary,
and in VP deletion constructions, at least one auxiliary must remain behind.
A coherent account of these two facts yields that the highest auxiliary must
move to I0.

Jill couldn't have been playing the piano but Bill could (have (been)).

Yes/No questions also move the highest auxiliary, but movement to
I0 does not change the word order. Something further is needed
for questions.

GB places two strong restrictions on movement. First, the only possible types
of movement are structure preserving or adjunction. Structure preserving
movement means that the moved element must correctly fit into the tree structure
which existed at D-structure. Heads can only move to head positions and phrases
can only move to specifier positions, since a complement that was not
subcategorized for cannot be added. Adjunction is allowed so the auxiliary
could possibly be adjoined to the IP, but the adjunction rule is recursive,
which incorrectly predicts that more than one auxiliary may be fronted
(38d). The second restriction is the principle of
'No Loss of Information,' meaning movement
is not allowed to a position which is already lexically filled. Thus, movement
of the highest auxiliary to
I0[+fin] is possible since the position
is not filled by a word at D-structure. But where does it go from there to
form a question?

The solution to this puzzle comes from embedded Yes/No questions such as
(41).

Embedded clauses are CPs, so main clauses are also assumed to be CPs. Clauses
containing a question have the feature [+q].

The D-structure for (38b), shown in
(42a), shows that both the
I[+fin] and the
C[+q] heads are available for movement.
(42b) shows the S-structure, with has filling
the C[+q] position and coindexed traces in both
the I[+fin] and
V[+aux] positions to show where it moved from.

The S-structure in (42b) meets the conditions on movement.
Further, this movement provides a natural account for the fact that no inversion
occurs in embedded Yes/No questions (41). In the embedded
clause, whether fills the C[+q] position,
so the principle of No Loss of Information
blocks the movement. A separate condition is not necessary.

Content questions provide examples of
-movement or movement to a
non-argument position. Argument positions are complements or the subject,
so a non-argument position is a specifier position other than the subject
or an adjoined position.

The question in (43) is generated similarly to the Yes/No
question above, with an additional movement added.

The D-structure in (44a) is almost identical to
(42a), except that which job is in the complement
position instead of the job and that NP is marked with the
[+wh] feature. The S-structure is generated by moving the
NP[+wh] to the specifier of
C[+q], along with the movement of the auxiliary
to I[+fin] and then to
C[+q].

The coindexed traces left after movement form a chain between the D-structure
position and the S-structure position of the lexical item. This chain allows
D-structure information such as subcategorization to be retained at S-structure.
It also accounts for proper assignment of semantic roles and case, as will
be discussed in sections 4.1 and
5.1.

The PC-PATR grammar parses the S-structure tree for each sentence. Since
questions involve movement of the auxiliary and a wh-phrase, null
elements are introduced for traces of the movements. The challenge of matching
the subcategorization information with the moved elements is handled by passing
features up or down through the tree.

PC-PATR allows null elements, but they must be constrained carefully so that
they only show up in the proper places. Therefore, various types of null
elements are used to model the GB traces. V-trace is used for the trace of
the auxiliary, NP-trace is used for the trace of a questioned NP, and PP-trace
is the trace of a questioned PP. The phrase structure rules which define
the null elements are given in (45a-c) and the rules
which allow them to be placed into the tree are shown in
(45d-f).[9]

In content questions, the trace in the D-structure position must be the correct
category as required by the subcategorization frame of the verb, and this must
also match the fronted element. The fact that an NP-trace must only be used
for verbs which subcategorize for an NP can be assured by the unification
constraint in (46a). Correctly matching the subcategorization
information with the fronted element is more complex. The constraints
(46b-e) assure that the appropriate subcat information
is placed into the
[moved:A-bar][10] list; that
the subcat list is stepped one position (as illustrated in section
2.2.2); and that the other information on moved
elements, as well as the subcategorization restrictions, is passed up.

To assure that the subcategorized complement matches the fronted element,
the contents of [moved] is passed all the way to the top. The
rule and constraints which assure the matching for a questioned NP are given
in (47). (A similar rule accounts for a questioned PP.)
(47a) requires that the NP be [type[wh:+]].
(47b) requires that the features on the NP unify with
the feature structure of the first element in the [moved:A-bar]
list. Then (47c) steps the list for the CP, whose
[moved] feature is required to be none for all
types of movement at the root level.

Head movement of the auxiliary to C0 provides an additional challenge.
Auxiliaries come in a certain order and each one requires that the verb following
it be in a certain form. This is accounted for by subcategorization for a
particular form of VP. In a question, however, the first auxiliary is in
C0 before the subject, not in I0 or V0 where
local subcategorization can take place. The V-trace is in the crucial position,
but the [subcat] information is up with the auxiliary. This
[subcat] information must be passed down to the position of
the V-trace.

For English it turns out to be a simple matter of placing constraints on
the appropriate phrase structure rules to assure that the
[subcat] information gets passed down to the rule containing
the V-trace, so that the complements can be checked normally. The phrase
structure rules and constraints that accomplish this are shown in
(48). Also shown are the constraints which assure that
the V-trace matches the moved V.[11]

While verb movement is quite restricted in English, in some other languages
it occurs in almost every sentence. This is true for VSO languages like
Quiegolani Zapotec (QZ). The analysis is that the D-structure is SVO order
with the subject in the specifier of VP rather than the specifier of IP position
(49a) (Black 1994,
following Koopman &
Sportiche 1991, McCloskey 1991). The verb
then moves to I0 at S-structure to obtain VSO order
(49b).[12]

The desire to maintain the stacked VP structure and the list notation for
[subcat] discussed in section 2.2.2
for QZ introduced an additional complication. Recall that the list is stepped
through as the complements are matched going up the tree from the verb.
Therefore, the [subcat] information needs to be moved down the
tree from I0 to the position of the V-trace also. The features
[down] and [up] were introduced to hold the relevant
[subcat] information. The information being passed down from
the verb in I0 is in [down subcat], while the linked
list of complements is being stepped through in [up subcat].
The two feature structures in [up subcat] and [down
subcat] are required to unify in the rule introducing V-trace. Sample
rules from the QZ grammar with the relevant constraints are shown in
(50).

Constructions such as passives, unaccusatives, and raising require movement
to the subject position. The constraints on movement discussed in the last
section still hold. In addition, semantic roles and the lack of subcategorization
for a subject are needed.

Passive constructions are the most well-known constructions involving movement
to the subject position. The old Transformational Grammar analysis begins
with a transitive deep structure, then creates a passive surface structure
by moving the subject to the by-phrase (or omitting it completely),
moving the object to the subject position, adding the passive be and
changing the verb form appropriately. This movement analysis of passives
captured the generalizations that:

The 'subject' of a passive verb corresponds to the object of its transitive alternant.

GB does not account for passives in exactly the same way as Transformational
Grammar did, since movement of the object to a previously occupied subject
position would violate the principle of No
Loss of Information. The generalizations in (51)
must still be accounted for, and GB goes a step further than
(51c) to account for the synonymy between the active
and passive sentences, as seen in (52).

General semantic roles, such as Agent, Theme, Recipient, Goal, Locative,
etc., can be linked to arguments within the lexical entries to capture this
synonymy. For example, the lexical entry for invite would
be:[13]

invite, V, [ _ NP (PP[to]) ]
| |
invite' <AGT, THEME, GOAL>

where each syntactic complement (called internal arguments) must be linked
one-to-one with a semantic role, and one additional role may be linked to
the external argument (or subject). Lexical
entries apply at D-structure, so not all verbs assign a role to the subject
position.[14]

Further assumptions about lexical entries in GB include the desire that the
related forms of a word share a single subcategorization frame and that there
should not be any cross linking of syntactic arguments and semantic roles.
For example - directly relevant to the analysis of Passive - the
THEME should not be assigned to the object
in one case and to the subject in a related entry. This assumption is formalized
under the Uniformity of Theta Assignment Hypothesis
(Baker 1988:46), allowing the broader claim that
the THEME role should always be assigned
to the direct object when it is assigned, since that is its position in normal
transitive verbs; the RECIPIENT role
is assigned to the indirect object, etc.

Though the semantic roles are assigned at D-structure through the lexical
entries, further assumptions forbid movement from changing the linking between
syntactic arguments and semantic roles. Therefore, movement
out of most positions is allowed, as we have seen with wh-question formation,
but nothing else can move into that position. The semantic role stays with
the original position rather than moving with the phrase; the semantic role
is not part of the tree but part of the lexical subcategorization that goes
with the D-structure position. Movement into a position linked to a semantic
role is not allowed, since it would cause the moved element to take on that
semantic role, and thus alter the original linking. The coindexed trace provides
the link between the moved element and the position it occupied at D-structure.
Therefore, both semantic role and subcategorization requirements are still
recoverable at S-structure.

Given these assumptions, the GB account of passive must be partly done in
the lexicon and partly by movement. A lexical rule, such as that given in
(53), is used to capture generalizations
(51a-b) that passive verbs are related to transitive
verbs. In order to account for alignment of the semantic roles between entries
as much as possible and follow the Uniformity of Theta Assignment
Hypothesis, the object NP remains in position at D-structure.

Note that no external argument is assigned
by the passive verb. The D-structure for the passive sentence is as shown
in (54):

Movement of the object to subject position is still needed. This movement
can take place since there is no semantic role linked to the subject position
at D-structure and the position was not lexically filled, so the principle
of No Loss of Information is not violated.
A coindexed trace is left behind to maintain the linking of the object to
its semantic role. So S-structure looks like (55):

The assumptions made about semantic roles in the lexical entries also requires
us to distinguish between types of intransitive verbs, following the Unaccusative
Hypothesis (Perlmutter 1978). This means, for
example, that while verbs like sleep would have an
AGENT in subject position at D-structure,
verbs like die would have an empty subject position at D-structure
with a THEME object, as shown in the
lexical entries. Movement of the
THEME to subject position for the
unaccusative verb die is exactly parallel to the passive movement.

sleep, V [ _ ] die, V [ _ NP ]
|
sleep' <AGT> die' <THEME>

The third type of A-movement (movement to an argument position) involves
raising predicates, such as seem and likely. These predicates
take either a finite or a nonfinite clause complement and do not assign a
semantic role to their own subject position. The lack of a semantic role
assigned to the subject position can be seen when there is a finite clause
complement by the presence of the dummy it(56a).
In the case of a nonfinite clause complement, the subject of the lower clause
must raise to the main clause subject position.

The implementation in PC-PATR accounts for the A-movement of the object in
passives and unaccusatives or the lower clause subject in raising constructions
to the main clause subject position in an analogous way to
-movement. These details are
briefly covered before moving to the issue of how the changes in the lexical
entries are made.

As far as the phrase structure rules are concerned, A-movement is handled
within PC-PATR in a completely parallel way to the other types of movement
discussed in section 3.2.1. The trace of A-movement
is distinguished from the -traces
by being called NP-A-trace. The moved category is stored by
the [moved:A] feature. The rule responsible for introducing
the A-trace for both passives and unaccusatives is given in
(57a), while the A-trace for raising is introduced by
rule (57b).

At the top of the tree, one of the options for the subject position requires
that no subject was subcategorized for by the predicate in that clause
(58a) and that the <moved A first>
path contains [cat:NP](58b). This option
accounts for the surface subject of all three types of A-movement.

The PC-PATR implementation differs from GB in that the syntactic form of
the subject is subcategorized for (although it is always either an NP or
none, which is consistent with GB). Passives, unaccusatives,
raising verbs, and auxiliaries have [subcat:[subj:none]] within
their feature structure and thus will have a subject only via A-movement.

Putting in the semantic roles seemed to just add the additional feature of
[role] for each NP, slowing processing without appreciable gain.
In order to use the semantic roles to rule out incorrect parses, we need
to specify which semantic roles a given lexical item or phrase can fill and
then unify this set of roles with the subcategorization requirements of the
predicate. Which phrases can be adjuncts and which must be complements must
also be restricted. These restrictions are not currently implemented; a negation
mechanism allowing specification of what a certain atomic value could not
be would be helpful.

The relationship between transitive and passive forms of a particular verb
is handled via disjunctive templates.[15]
For example, a verb like broken would have the feature description
+en trans/pass, while the form broke is marked +past
trans/unacc. The relevant templates are given in
(59). These templates are then matched with the number
of arguments present in the sentence, etc., to determine whether a correct
parse can be given.

As mentioned above, this same implementation of passives and unaccusatives
would be possible without the addition of semantic roles at all for English.
Semantic roles are more interesting and necessary in the implementation of
a syntactic parser for Tatana' in Malaysia, where the case of each nominal
is morphologically marked and the voice of the verb determines the required
ordering and semantic role of the
arguments.[16] The complex disjunctive template
for Patient voice (one of three voices: Actor, Patient, Referent) is shown
in (60). This template unifies with the
[trans], [intrans], or [ditrans] templates
on a particular verb to limit the options available. Note that the basic
word order in Tatana' is VSO, but a nominative NP may be fronted before the
verb.

In the areas of both case and agreement, English is deficient compared to
some other languages. For both GB and PC-PATR, the features are assumed to
be present, whether or not there is distinct morphological marking.

GB assumes that all NPs must have Case (abstract,
not necessarily morphological case) at S-structure. This is expressed by
the Case Filter (61). Nominative Case is assumed to be
assigned to the subject position (the specifier) by
I[+fin]. Similarly, Genitive Case is assigned
to the possessor position (the specifier) by the noun. Accusative Case is
assigned by a verb to its NP complements, by a preposition to its complement,
and by the complementizer for to the subject (the specifier) of the
nonfinite clause which follows it.[17]

The Case Filter, coupled with Burzio's Generalization
(Burzio 1986) that predicates that do not assign
a semantic role to their external argument
(i.e. do not subcategorize for a subject) cannot assign Case to their
complements, provides the motivation for movement to the subject position
for passives, unaccusatives, and raising constructions. These NPs cannot
receive Case in their D-structure positions, so they must move to a position
which assigns Case: the specifier of
I[+fin].

Agreement is also seen to be linked to a particular position or to a relationship
between positions in the tree structure. An
I[+fin] agrees with its specifier (the subject),
and it can also subcategorize for a VP with the proper agreement features
(including person, number, and tense).

The case and agreement features of lexical items, such as
[person], [number], [gender],
[case], and [vform] (for verb form, including tense),
are introduced as subfeatures under [agr] via templates. Structural
case and structural agreement are then constrained via the phrase structure
rules. For example, the constraint <NP agr
case>=nominative on the rule which introduces subjects requires
that the subject is in nominative case. Similarly, NP objects are required
to have accusative case via a constraint on the rule V' = V'_2
NP. Agreement between the subject and the verb is enforced by percolating
the [agr] features on the verb up to the I', and then requiring
that <I' agr>=<NP agr>.

The implementation of case and agreement restrictions becomes more challenging
in dealing with questions. In content questions, the fronted wh-phrase
retains the case it would have had in its D-structure position. This is enforced
at the point of the trace by placing the correct case information within
the [moved:A-bar] feature matrix, so that this
[case] feature will have to unify with the [case]
of the fronted NP. The rules and relevant constraints for subjects and objects
being questioned are shown in (62).

The second problem with questions is dealing with the agreement after subject-aux
inversion. The verb agreement features are not known at the I' level, as
they are in non-questions, since the verb that agrees with the subject is
up in the C position. The [agr] features of the subject are
therefore passed up to the IP for questions, so that agreement can be enforced
in the rule C' = V IP via the constraint <IP
agr>=<V agr>.

Binding constructions are another instance of non-local restrictions on
agreement. The PC-PATR implementation of these restrictions is discussed
in section
6.2.[18]

We have now accounted for cases of movement, which GB handles via coindexed
traces, by feature passing in PC-PATR with unification constraints at both
ends. The binding of reflexives and equi constructions can be modeled in
a parallel way, since coindexing is again involved.

Binding theory seeks to provide an account of where a reflexive versus a
pronoun versus a full NP is allowed or required. Relevant examples are given
in (63), where intended coreference is indicated by
subscripts.

a. a does not dominate
b, and
b. the first branching node that dominates a
also dominates b.

aBINDS b iff

a. a c-commands b, and
b. a and b are coindexed.

a A-BINDSb iff

a. a is in an argument position, and
b. a binds b.

In the tree (64)Sally and Max are both
in argument positions and they both c-command the object of disliked
and are coindexed with it. Therefore the object of disliked is A-bound.

The principles of Binding Theory determine whether a pronoun or a reflexive
anaphor or a full NP is correct in a particular position. Informally, Principle
A says that an anaphor can only be used when the position that A-binds it
is local enough (usually within the same finite IP): Max is close
enough to the object of disliked so that the anaphor is correct, but
Sally is too far away to use herself.

Principle B says that a pronoun can only be used if it is not A-bound at
all, or if its A-binder is far enough away. This is why him cannot
be used to refer back to Max but her may refer back to
Sally.

Finally, Principle C says that nonpronominals may not be A-bound in the sentence
at all, which rules out repetition of full nominals.

The Principles of Binding Theory thus partition nominal phrases into three
different types. These partitions are characterized by the two features
[±anaphoric] and [±pronominal], where pronouns are
[-ana,+pro], reflexives and reciprocals are
[+ana,-pro], and full nominal phrases are neither pronouns
nor anaphors so they are [-ana,-pro].

The chart in (66) shows these featural distinctions and
which Principle of Binding Theory applies. Empty categories are also included,
since GB claims that the chain coindexing established by movement is equivalent
to the coindexing in binding relationships between overt nominals. Four types
of empty categories are recognized, corresponding to the four possible feature
specifications.

Section 4 covered the trace of A-movement in passive
and unaccusative constructions. By saying that this trace is anaphoric and
subject to Principle A, the movement is restricted to local domains. The
trace of -movement in the formation
of content questions was introduced in section 3.
This trace is subject to Principle C in that it cannot be bound by an element
in an argument position. Since
-movement is movement to a
nonargument position, this requirement is clearly met.

The two new empty categories are not traces, but empty elements in the lexicon.
pro is the empty pronoun allowed in pro-drop languages, usually
because of agreement morphology on the verb to specify the person and number
of the subject, e.g. pro No hablo español. This empty
pronoun shows up in all the same places that an overt pronoun does and is
therefore subject to Principle B.

Finally, PRO is the empty subject in non-finite clauses, sometimes
called controlled PRO. Since it is both anaphoric and pronominal,
it is subject to both Principles A and B of Binding Theory. From this is
derived the fact that PRO must be ungoverned and therefore also not
receive Case.[19] The only legal position
for PRO (and only PRO can be there) is shown in
(67).

Both pro and PRO have intended reference, even though they
are null. Since they are referential, they bear a semantic role, unlike the
dummy it, which has phonetic content but is nonreferential.

English does not use pro, but we can see the use of PRO in
equi constructions, such as:

Note that both try and want assign the semantic role of
AGENT to their external
argument (subject), in contrast to raising predicates like seem
and likely. The verbs in the lower clause, fix
and win, also assign AGENT roles to their
external argument. GB requires a one-to-one
mapping between NP positions and semantic roles, so a null category is necessary
in these constructions, namely, PRO. The tree for
(68a) is given in (69).

Within PC-PATR, only the Principle A requirement that an anaphor agree with
the closest subject is implemented. It is not possible to enforce the
requirements of Principles B and C that a pronoun or full NP does not agree,
at least for third person pronouns. If negation capabilities were added to
PC-PATR, it would be easier to restrict against two first or second person
pronouns as subject and object of the same clause.

In order to account for the reflexive agreement, [gender] features
are added to all of the nouns, pronouns, and reflexives in the lexicon under
the [agr] feature discussed in section
5.2. Reflexives are further distinguished by the
feature [type:[anaphor:+]]. All of the phrase structure rules which
could introduce an anaphor (including NP objects of V and objects of prepositions
which are themselves objects of either V or N) are also constrained to place
the [agr] features for the anaphor into the [bind]
feature matrix, which then acts in a parallel way to the [moved]
feature matrix. The [bind] feature percolates as far
as the closest
subject position to enforce coreference, or unification of the
[agr] features.

The most interesting case is the interaction between reflexives and equi
and/or raising constructions. These are the only cases which allow apparent
long-distance binding of anaphors. However, as shown in
(70), the coindexed PRO or A-trace serves as the
local A-binder of the reflexive and then it takes its reference from the
upper clause subject.

Equi constructions, such as (70b) are handled in PC-PATR
via the null category PRO, which is introduced into the grammar only
by the rule in (71). This rule is restricted to embedded,
nonfinite, non-question clauses (71a-c). The subject
subcat list is stepped (71e), since PRO counts
as the subject of the embedded predicate. The crucial constraint allowing
the apparent long-distance binding of reflexives is
(71f), where the [bind] features are percolated
up to the IP. This is in contrast to the regular subject rule
(72) (used also for the upper clause of an equi or raising
construction) where the [bind] features must unify with the
[agr] features on the overt NP subject.

This documentation of work in progress is instructive in at least two ways.
First, it presents the development of a parser in PC-PATR that accounts for
a broad range of data, illustrating the capabilities of the PC-PATR program.
Secondly, the attempt to implement GB theory computationally forced precision
into the analysis at several points. For example, the implementation showed
that 'coindexing' and feature sharing of nonlocal constituents must be passed
through all local subtrees, including intermediate projections, whereas GB
states only that heads and their maximal projections and heads and their
specifiers may share features.

5 The order of the elements on the right
side of these phrase structure rules is dependent upon the basic word order
in the language.

6 Both C0 and I0
are functional heads rather than lexical heads. More recently, additional
functional heads such as Neg0, Agr0, and
Tense0 have been proposed to further break down I0
(Pollock 1989) and D0 has been proposed
as the functional head of the noun phrase, taking NP as its complement
(Abney 1987, Stowell 1989).
None of these additional functional heads (or the phrases they project)
are used in the English implementation in PC-PATR, though the implementation
for Quiegolani Zapotec requires a NegP phrase for negation between CP and
IP and uses the DP structure for nominal phrases.

7 The original PATR-II formalism allowed
X for subcategorized complements (Shieber 1986:72).
While the X is desirable in capturing the generalization that whatever follows
must be a complement determined by the subcat information, it turns out to
not be feasible in a fuller system due to the differing constraints needed
for the various complements.

8 GB does not allow subcategorization
for subjects directly, but only for whether or not there is a semantic role
assigned to the subject (see section 4.1). In the
PC-PATR implementation, I allow subcategorization only for an NP subject,
which will have a semantic role, or none.

9 In the current form of the grammar,
the highest auxiliary is assumed to begin in the I0 position,
rather than having to move there, so V-trace fills only that position for
questions (45d). For declarative sentences with an auxiliary, the highest
auxiliary is placed in the tree via the rule I' = V VP,
constrained by <V type aux> = +.

10 The [moved] feature is
a personal innovation, based upon the slash features of GPSG
(Gazdar, Klein, Pullum & Sag 1985). Following
GB, [moved] has three subordinate features: [A-bar]
(for wh-movement), [head] (for head movement such as
movement of the auxiliary), and [A] (for argument movement to
be discussed in section 4).

11 Rules (48a & c)
are restricted to use in root clause questions only, by other constraints
not shown here. (48b) is a general rule for the position
of the subject as the specifier of IP, but the constraints listed here are
included only under the root clause question option for this rule.

12 In the GB analysis, the aspect marking
is in I0 at D-structure and the morphological subcategorization
requirements motivate movement of the verb to adjoin to I0 (see
Black 1994, Rizzi &
Roberts 1989). For PC-PATR, only the position of full words at S-structure
is modeled. Therefore, the verb with its aspect marking is shown in
I0 at S-structure.

13 The line containing the semantic roles
is introduced by the logical semantic predicate which is indicated by the
syntactic verb form followed by a prime. The lexical entry thus specifies
both syntactic and semantic subcategorization information.

There is only one external argument =
John. Which verb assigns a semantic role to John? It is the
AGENT of the verb hit; a home
run is the THEME of hit.
Auxiliaries do not assign external arguments:
there is nothing anyone did or experienced to may. Instead they simply
select a VP complement that is a type of
EVENT or
STATE.

15 The PC-PATR documentation includes
discussion of a Lexical Rule. Unfortunately, the GB lexical rule assumes
different output conditions than PC-PATR does, so I could not use that mechanism
for handling passives in GB.

16 The information on Tatana' was obtained
from John Dillon at the Computational Linguistics workshop at the 1994
session of the Summer Institute of Linguistics held at the University
of North Dakota.

17 These positions which receive Case
are the positions that the particular heads govern.

18 An area where agreement is particularly
tricky is in coordination constructions. The current grammar restricts coordinate
constructions to third person, plural agreement. Much more would need to
be added to account for the full distribution of agreement patterns in coordinate
constructions.

19 This is based upon the formal definition
of the local domain of Principle A, called the governing category.